(1) Field of the Invention
The present invention relates to a process control stabilizing system which is incorporated in an image forming apparatus and controls the electrophotographic process so as to provide an image in an optimum condition.
(2) Description of the Related Art
Generally, with regard to photoconductive members for use in image forming apparatuses, the surface potential of a photoconductive member varies largely dependent upon environmental conditions of the location where it is used. As regards OPC's (organic photoconductors), for example, the surface potential at low temperature environment is lower by 100 to 150 V than that at a normal temperature due to dependance of the mobility of photocarriers therein. With regard to Se-photoconductors, the amount of thermally excited carriers varies dependent upon temperatures, so that the potential increases by 50 v at a low temperature and decreases by 50 to 100 volts at an elevated temperature. The temperature dependence is problematic. Further, the OPC's exhibit a tendency that the thickness of the photosensitive layer thereof is reduced by mechanical stresses (that is, scratching or abrasive effects by a cleaner blade and/or copy paper) with the total copy volume increased. The variations of the surface potential in a photoconductive member due to the aforesaid effects, would lower the density of an image copied or bring about other great deterioration to image quality. Further the variations would influence the toner consumption amount for development to give rise to a waste of toner. On the other hand, as to developers, the amount of frictionally generated static electric charges of toner varies depending on environment charges. Specifically, under a circumstance at a low temperature with low humidity, toner tends to acquire more charges resulting in lowering in image density (i.e. the halftone density lies within approximately 0.8.+-.0.4), while toner powders get less charges at an elevated temperature with a high humidity, to induce increase of the image density, deterioration of gradation reproduction behavior, and/or a waste of toner etc.
Moreover, even if the copying mode were changed, the effect would sometimes fail to reflect due to the disadvantages stated above. That is, the image quality between different modes would not be clear or distinguishable, or the intention to lessen the toner consumption could not be achieved. In order to eliminate these defects and drawbacks, there are disclosed various proposals of process controls one of which is cited as Japanese Patent Publication Sho 61 No. 29502. A method of the publication includes the steps of measuring electric charges in both dark and light portions and then controlling the condition for charging based on the measurement in the dark portion while controlling the condition for exposure or development with reference to the measurement in the light portion, in order to provide exactly controlled images.
Another method includes the steps of detecting a surface potential of a photoconductive member at proper times by surface potential meter provided inside copier and adjusting optimally based on the detected quantity the power of charger and/or the applied voltage for exposure lamp. Still another method includes the steps of exposing an image of a referential white board etc. onto a photoconductive member, developing the latent image into a visual image with toner, measuring the density of the toner-image by an optical sensor, and optimally adjusting based on the detected quantity the power of charger, the toner density of developer, the bias voltage for development, and the power voltage for exposure lamp.
With the conventional arrangement described above, however, it has been impossible to get proper information on images from sensors due to a lack of uniformity in discharging by a charger. The charger is at all times exposed to and polluted with toner powders splashing inside the copier, an evaporated and splashed silicon oil used in heat-fixing process, and/or any other dirt outside the copier. This makes it very difficult to keep discharge of the charger uniform across the longitudinal direction of a photoconductive member. For this reason, unevenness in discharging occurs and this causes the photoconductive member to have an uneven distribution of its surface potential in the longitudinal direction thereof, or the toner-developed image on the photoconductive member for the referential white board to become uneven. If a surface potential meter or optical sensor samples the portion with such unevenness, the resultant measurement cannot represent a cross-section or typical value of the entire system, and the process of the system might disadvantageously be controlled based upon the erroneous measurement. This failure to control the process may possibly bring about various serious problems. That is, a process condition widely deviates from an optimally controlled condition might cause fluctuation of the quality of images. A defectively controlled process condition might damage the photoconductive member. Alternatively, an unusual increase in toner density fails to provide balanced frictional electrification charges to toner powders, yielding weakly charged toner which would make images foggy. Moreover, augment of toner might cause the toner powders to splash, polluting the inside of the copier.
To overcome these problems, that is, to measure against the uneven distribution of static electric charges on the photoconductive member, various proposals have been made. For example, Japanese Patent Application Laid-Open Hei 2 No. 179659 discloses a corona charging device in an electro-copier able to detect the unevenness of charges and automatically executing cleaning operation of a corona charger when the charger causes uneven discharge. The device detects irregularity of toner density of the image on a photoconductor as an indicator for unevenness of discharging by the corona charger. More specifically, if a toner density sensor incorporated in the device exhibits an output power lower than a threshold level, the device recognizes occurrence of the unevenness in discharging. Based upon the detection, cleaning means is activated which comprises cleaning pads sandwiching the charging wires stretched in the charger and being slidable so as to be driven by a stepping motor. Thus, on the occasion of detecting uneven toner density, the stepping motor is activated to execute cleaning operation.
Like the above disclosure, Japanese Utility Model Application Laid-Open Hei 3 No. 20349, relating to an image forming apparatus equipped with an automatic cleaning mechanism in a corona charger, measures the density of a referential image on its photoconductive member by using detecting means comprising plural photosensors, and executes cleaning operation in the same manner as described of the above prior art by moving and sliding the cleaning mechanism when difference between the density values detected by the different photosensors is found to exceed a predetermined level.
Another Japanese Utility Model Application Laid-Open Hei 2 No. 123947 proposes a technology relates to an image recording apparatus comprising a plurality of developing units, wherein charging wires in the charger are cleaned by a cleaning means every time a different developing unit is selected. Here the cleaning means used is of slidable type with the same structure described above. The object of this apparatus is to conduct cleaning operation of the charging wires in accordance with the selection of the developers.
Of these three publications, the first and second articles disclose the apparatuses all of which execute cleaning operation of the charging wires in the charger based on the comparison of the output power of the toner density sensor or sensors with a predetermined value. In consequence, it is true that the charging wires are cleaned effectively, but it is not that the process is controlled exactly based upon the density change, so that these cannot be thought of as the most suitable process control methods.
The third article discloses a technology in which the charging wires in the charger are cleaned before a currently engaged developing unit is replaced by a different unit in order to change the developing process. Therefore, this method is not the one that keeps on controlling a certain developing unit with reference to its output information.
On the other hand, Japanese Patent Application Laid-Open Hei 3 No. 105360 discloses an image forming apparatus capable of performing automatic maintenance including cleaning operation of charger in parallel with the operations for attaching and detaching an IC card as a portable external memory means. Cleaning means disposed slidably with sandwiching corresponding electrodes is provided for each of a primary charger, a transfer charger and a separation charger, and is adapted to be driven by a respective motor supplied by a motor driving power source. The activation of these motors, in consequence the cleaning operation, is determined by a counter for total copy number. Here, the connector of the aforesaid IC card serves as a detecting means, being connected detachably to the apparatus body.
The method is to control automatically certain subjects for maintaining the image forming apparatus using the IC card. Nevertheless, in this method, the wires of the chargers will not be cleaned until the detecting means sends out the order i.e, the signal for cleaning. In other words, the method is not the one that carries out an exact process control following the information from the photoconductive member.